A thermal storage medium is a material which contains a substance with a high heat capacity, and can store heat in the substance and release it as needed. Thermal storage media are used in various fields including air-conditioning units in facilities such as hotels, airports and underground cities, automobile canisters, electronic parts such as IC chips, underwear fibers, thermally insulated containers for transportation of organs, curved mirrors and bridge concrete materials.
Water is commonly used in thermal storage media. In most cases, however, thermal storage with water depends solely on the use of sensible heat caused by a temperature change. Thus, attention has been drawn to thermal storage media that use not only sensible heat but latent heat caused by a phase change.
Paraffins are compounds that can use a latent heat generated by a phase change. However, paraffins are dangerous when used in the liquid state. In order to ensure that a paraffin does not leak when it has become fluidized, it is necessary that the paraffin be contained in a closed container or a bag. In such cases, the use of containers or the like having sufficient strength is not viable because costs are increased. On the other hand, a simplified container or the like is readily breakable to cause the risk that a paraffin will leak or come out during a long term of use.
Thus, approaches other than the use of containers have been increasingly adopted such as impregnating a porous substance with a paraffin, microencapsulating a paraffin, and a combination of these methods. However, even such methods have a problem in that the leakage of a paraffin from a porous substance or a microcapsule cannot be completely prevented. Further, these methods entail complicated production steps and a cost increase, as well as a problem that the content of a thermal storage material per unit volume is reduced. In particular, microencapsulating a paraffin has a problem in that the thermal storage performance per unit volume is decreased when a paraffin fails to completely fill the space in the microcapsule.
Another conventional method is to knead a paraffin into a crystalline polyolefin such as crystalline polyethylene. However, crystalline polyolefins are hard and are difficult to handle. Further, at high temperatures, a paraffin is phase-separated and bleeds out from a crystalline polyolefin. To prevent this problem, a solid container needs to be used. Thus, such a method is not viable.
In order to overcome the problems described above, thermal storage media comprising a paraffin and a thermoplastic elastomer as main components have been disclosed (see, for example, Patent Literature 1). Such a thermal storage medium has a latent heat as high as 30 kcal/kg or more (126 kJ/kg or more), and preferably 35 kcal/kg or more (147 kJ/kg or more) at service temperatures, which is free from phase separation or paraffin bleeding even at or above the maximum crystal transition temperature of the paraffin contained (hereinafter, also referred to as “Tmax”, corresponding to the melting point in most cases), and does not exhibit brittleness even at or below Tmax (below which the paraffin becomes solid) and shows appropriate flexibility without being broken even when formed into a sheet. Examples of the thermoplastic elastomers described in Patent Literature 1 include styrene block copolymer elastomers and olefin elastomers.